The direct antiproliferative effects of interferons will be studied in human cells in tissue culture. Cell lines sensitive to interferon and resistant variants derived from these lines will be studied in parallel. The time of inhibition of DNA synthesis in cells treated with interferon will be determined by incubation with 3H-thymidine. Cell extracts will be prepared at the same time from treated cells and the activity of interferon-induced enzymatic activities will be determined. An increase in the activity of two enzymes, a protein kinase and an oligoadenylate (2-5A) polymerase, has been reported in human cells treated with interferon. If an elevation of either of these enzymatic activities precedes the inhibition of DNA synthesis in cell sensitive to the antiproliferative effects of interferon, these studies will be extended to the interferon-resistant cell lines. In particular, the reasons for the high sensitivity to growth inhibition of the human lymphoblastoid cells Daudi will be investigated in a systematic way. Among the working hypothesis to be tested are the presence of an RNA which can activate the enzyme 2-5A polymerase in Daudi cells, a low level of the enzyme which degrades 2-5A (2'-phosphodiesterase), or a high level of endonuclease activated by 2-5A. The effect of 2-5A accumulation in intact cells will be studied by following the possible degradation of labeled cytoplasmic RNA. Other experiments will investigate the increase in the basal level of 2-5A polymerase in lymphoblastoid cells treated with glucocorticoid hormones or growth-inhibited by nutrient deprivation, that have been observed in preliminary studies. Finally, analogs of 2-5A modifield in the adenine or methylated in the ribose moiety will be tested for 2-5A-dependent endonuclease activation in vitro and for inhibition of protein synthesis in intact cells.